Technicians responsible for maintaining optical fiber telecommunications systems need optical couplers which can couple optical signals into and out of unbroken optical fibers. The technicians use such coupling devices to identify fibers which are carrying live telecommunications traffic without interrupting such traffic, and to locally launch and detect optical signals during splicing operations.
One type of optical coupler used for such operations comprises a bending element and a coupling element. The bending element and the coupling element have complementary surfaces which define a desired bend profile for an optical fiber which is held between the two complementary surfaces The coupling element is at least partially transparent and includes an optical source or detector which is aligned with the bend in the fiber so as to be optically coupled to the bent fiber.
In some couplers of this type, both the bending element and the coupling element are rigid bodies. In these structures, any mismatch of the complementary surfaces which define the desired bend profile may permit deviation of the fiber from the desired bend profile, thereby degrading the optical coupling characteristics. Such mismatch may also permit movement of the fiber between the complementary surfaces, causing unstable optical coupling characteristics. Moreover, such mismatch may result in highly localized pressures which can permanently deform or damage a coating of the fiber. Thus, to achieve optimum coupling characteristics in these structures, the complementary surface profiles of both the bending element and the coupling element must meet very tight dimensional tolerances, and this may be difficult and expensive to achieve
In other couplers of this type, the bending element is a rigid body and the coupling element is a relatively deformable body. In these structures, any mismatch in the complementary surfaces which define the desired bend profile is resolved by deformation of the coupling element when the fiber is held between the complementary surfaces Thus, the surface profile of the rigid bending element effectively controls the bend profile of the fiber, and the dimensional tolerances on the surface profile of the coupling element are relaxed somewhat Because the coupling element deforms to conform to the surface profile of the bending element, the fiber is held tightly between the bending and coupling elements over a greater portion of its length, reducing movement of the fiber and improving the stability of the coupling characteristics. Moreover, deformation of the coupling element spreads localized pressures to reduce permanent deformation of the fiber coating.
Unfortunately, in optical couplers having a rigid bending element and a relatively deformable coupling element, the position of the fiber bend relative to the optical source or detector is not precisely determined, and this may degrade the reproducibility of the optical coupling characteristics. In particular, deformation of the coupling element will move the fiber bend closer to the source or detector, thereby changing the coupling geometry and the resulting coupling characteristics. In successive applications of the coupler to the same fiber or to different fibers, different amounts of compressive force may result in different coupling characteristics.
The problems of known optical couplers as described above are particularly acute when the couplers are used with single mode fibers in applications where relatively weak but reproducible coupling is required, and the coupling must be essentially the same over a broad range of wavelengths. For example, portable test instruments used to identify optical fibers carrying live telecommunications traffic without disrupting that traffic must reliably and reproducibly couple only about 0.5 dB from a single mode telecommunications fiber. If the coupling is much less than 0.5 dB, the instrument may fail to detect live traffic, and a maintenance-induced service outage may result. If the coupling is much more than 0.5 dB, the instrument may disrupt the live traffic and a maintenance-induced service outage may result. Single mode optical fibers can carry a great deal of telecommunications traffic, so such service outages are very serious. Optical signals carried by such fibers may have a wavelength as short as 1200 nm or as long as 1600 nm, so the coupling characteristics of the test instrument must be roughly constant over this wavelength range. Moreover, the coupling characteristics must be roughly constant over the full range of fiber dimensions and over successive measurements on the same or different fibers. The optical couplers described above cannot readily meet these stringent requirements.